One of the most important steps in the manufacture of high quality printing papers, coated or uncoated, is the calendering of the paper web to impart gloss and smoothness to its surface. A number of different processes exist for calendering paper. One long used for producing the highest quality product is supercalendering. Gloss calendering is another process, which while not producing the quality of supercalendering, does have process advantages over supercalendering. Substrata thermal molding is a recently developed calendering process which combines process advantages of gloss calendering with the quality advantages of supercalendering.
Substrata thermal molding is described in U.S. Pat. Nos. 4,624,744 and 4,749,445, which patents are hereby incorporated by reference. It employs a calendering nip formed by a heated metal roll and a resilient pressure roll. The metal roll is heated to a temperature higher than that employed in gloss calendering equipment. The required temperature varies with changes in the process conditions, but it will typically be in excess of 300.degree. F. on the surface of the metal roll. Very high nip pressure is also required, in excess of 2000 psi, which requires nip loads in excess of 1000 pli of calender width and typically between 1500 to 2000 pli.
The heated calendering roll for gloss calendering is typically a hollow thin walled metal cylinder or drum which is heated internally with steam at temperatures up to about 350.degree. F. The drum is made from chilled iron, ductile iron or chrome plated ductile iron, which provides a hard, abrasion resistant surface which takes and holds a high polish. Chrome plated drums provide excellent polished surface, but are easily scratched in operation. The gloss calender drum has been found satisfactory for calendering at the moderate temperatures and pressures of gloss calendering, but not at the conditions necessary for substrata thermal molding. The calendering roll rigidity needed for the high nip loads of substrata thermal molding require rolls with much thicker circumferential walls, in excess of 4 inches thick. The higher temperature requirements of substrata thermal molding have placed additional requirements on the heated roll. Much hotter internal heating fluids are required. This has required the use of higher boiling point fluids, such as oils. The hotter internal fluid temperatures require placement of fluid conduits close to the surface of the roll to decrease the thermal heat flow resistance to the roll surface. The use of multiple nips on each heated roll, which is advantageous in practicing substrata thermal molding, further increases the thermal requirements. It has been found that a second nip can increase the heat load by about 30% over that required for a single nip.
One form of roll employed in substrata thermal molding is the Tri-Pass roll produced by SHW Corporation. The Tri-Pass roll is a chilled iron roll with a very thick cylinder wall, typically about 7 to 11 inches thick. Holes are drilled axially through the roll close to the surface to act as conduits for the heating fluid. Chilled iron rolls have an outer layer of hard white iron, an inner structure of cast gray iron, and an in between layer of mottled iron with properties between white iron and gray iron. The fluid holes are preferably drilled through the softer gray iron, so they are positioned as close to the interface of the mottled iron and the gray iron as possible. The thickness of the white iron is typically from 1/4 inch to 3/4 inch and the thickness of the mottled iron is typically from 1 inch to 11/2 inch. This places the holes about 2 inches from the surface. In addition to the disadvantage of requiring placement of the holes further from the surface, the mottled iron makes it difficult to drill the holes straight because of an irregular location of the interface.
The thermal conductivity of the white iron (13 BTU/Hr.Ft..degree.F.) and mottled iron (17 BTU/Hr.Ft..degree.F.) is lower than of the gray iron (25 BTU/Hr.Ft..degree.F.), providing an advantage and a disadvantage in conducting heat to the surface. Heat has to travel farther from the holes to the surface at points on the surface between the holes. The lower conductivity of the white iron and mottled iron moderates the variation in the surface temperature, which in turn provides more uniform finishing to the paper. However, the lower conductivity, coupled with the extreme thermal requirements of substrata thermal molding, creates a very large temperature drop from the heating fluid conduits to the surface of the drum. Drops of about 150.degree. F. and higher can occur in commercial operations. The much higher temperature in the interior of the roll creates greater thermal expansion than the lower temperature on the surface, resulting in the creation of high tensile hoop stresses. The hoop stresses can be great enough to exceed the ultimate tensile strength of the chilled material and destroy the chilled iron roll Only lower temperatures or slower operating speeds permit safe operations. The safe operation limit is below 7,000 BTU/Sq.Ft./Hr. heat flow through the rolls for chilled iron rolls. One solution for reducing the heat flow through the roll is to provide part or all of the required heat to the surface of the roll from external sources, such as induction heating the surface. Externally mounted devices, however, are not completely satisfactory. They are not energy efficient. They do not provide uniform surface temperature over the widths of commercial size rolls. They create impediments in the path of the paper web and additional operating problems. An internally heated roll would be more satisfactory if the hoop stresses can be kept to an acceptable level for the material chosen.
Substituting for chilled iron rolls is not easy. The advantageous properties of the white iron surface are not easy to find in other materials. The surface must be capable of developing a high polish. It must be sufficiently hard to resist deterioration of the polished surface when faced with abrasive paper coating materials, the abrasive action of a cleaning doctor blade, and a corrosive environment. It must have the surface characteristics necessary to release cleanly the paper and coating after calendering, such as an appropriate surface energy and polarity component.